RF (Radio Frequency) electricity has been used for decades to cauterize and coagulate tissue in surgical procedures. Devices used to apply RF energy to tissue fall generally into two categories: mono-polar and bipolar. Bipolar electrosurgical instruments incorporate both active and return electrodes into the surgical instrument, substantially restricting the flow of electric current to tissue that is placed between the electrodes. In mono-polar electrosurgical instruments, on the other hand, the return electrode is placed outside the patients body, on the patients skin. Thus, in a mono-polar electrosurgical instrument, current flows from the active or treatment electrode through the patients body to the return electrode. Both mono-polar and bipolar electrosurgical instruments rely, at least in part, upon resistance heating to treat (e.g. cauterize and/or cut) tissue. As current is passed through tissue, the electrical resistance of the tissue results in the dissipation of power in the form of heat. As the temperature of the tissue rises, its characteristics, including electrical resistance, change. When the tissue temperature reaches approximately 67-70 degrees C, coagulation begins. As additional energy is dissipated in the tissue collagen, which forms the backbone of the tissue matrix, continues to break down and "melt". Once the collagen begins to break down, the tissue begins to coagulate. When the collagen begins to break down, compression of the tissue will cause the compressed tissue layers to fuse, sealing adjacent blood vessels. When the tissue temperature reaches one hundred degrees C most fluids (including water) are driven off or evaporated from the tissue, desiccating the tissue and substantially increasing the electrical resistance of the tissue. The desiccated tissue may then be cut or separated with little effort. The rate at which energy is dissipated into tissue is dependent on many factors, including the electrical resistance of the tissue and the density of the electric current flowing through the tissue. Since electrosurgical instruments are generally designed to be used to treat a variety of tissue types, current density becomes an important design consideration, and, particularly in bipolar electrosurgical devices, current density is, for a particular tissue type, a function of the number, size, shape and placement of the device electrodes.
In many surgical applications, it is desirable to use bipolar electrical energy as a means of cutting and/or coagulating tissue. In bipolar electrosurgical instruments, it is generally desirable to ensure that the flow of electric current is confined to the tissue in the instrument and, to a significantly lesser extent to the tissue adjacent the instrument. Generally, in prior art bipolar electrosurgical instruments, these goals have been accomplished by designing an instrument which grasps or clamps the tissue prior to the application of electrosurgical energy. Such bipolar electrosurgical instruments are well know in the art and, in particular, many designs have been suggested for surgical instruments which coagulate tissue either prior to cutting the tissue or during the cutting process. In most of these instruments, the tissue is first grasped by jaws which apply pressure to the tissue prior to the application of electrosurgical energy. In such instruments, the grasping jaws either constitute or include the electrodes which supply the electrosurgical energy, although, in some designs, one or more of the electrodes may be incorporated into other elements of the instrument, including, for example, the cutting element. Thus, in such bipolar electrosurgical grasping instruments, the tissue being treated is first grasped, then electrosurgical energy is applied by the electrodes, then the tissue is cut or separated, and, finally, the tissue is released and the grasping instrument is moved to fresh tissue so that the process can be repeated. While this procedure is very effective in many surgical procedures, when working in certain types of tissue, such as mesentery tissue, it may become tedious to continuously grasp and release as the instrument is moved through the tissue. However, since tissue such as mesentery tissue is vascular and will bleed if the blood vessels are not adequately sealed, it is important to ensure that the blood in the tissue on either side of the cut line is thoroughly coagulated prior to separating the tissue. Further, since many modem surgical procedures are performed in very small spaces, there may not be sufficient room to use an instrument with jaws which must be opened after each application of electrosurgical energy.
It would, therefore, be advantageous to design a bipolar electrosurgical instrument adapted to coagulate and cut tissue while moving continuously through the tissue. It would further be advantageous to design a bipolar electrosurgical instrument adapted to coagulate and cut tissue, including vascular structures, while moving the instrument continuously through the tissue, wherein the coagulation region is substantially confined to the width of the jaw assembly. It would further be advantageous to design a bipolar electrosurgical instrument adapted to continuously receive, coagulate and divide the coagulated tissue as the electrosurgical instrument is moved through the tissue. It would further be advantageous to design a bipolar electrosurgical instrument adapted to continuously receive, coagulate and divide tissue wherein the electrosurgical current through the tissue in substantially self limiting.